1. Field of the Invention
The invention relates to an electrical switch apparatus employing a piezoelectric element to detect a mechanical pressure signal, and using the resultant voltage generated by that element to accomplish a switching function. The apparatus operates without any appreciable mechanical deflection and the invention is particularly adapted for and particularly useful in platform scales for the energization of a scale reading indicating system. In particular, it is especially useful in platform scales which operate with a very low deflection in the weight measurement system such as the scale described and claimed in a concurrently filed copending U.S. patent application Ser. No. 209,740 (6HW-5466) filed by Michael J. Ostrelich for "Thick Film Resistor Force Transducers and Weighing Scales" and assigned to the same assignee as the present application. The present invention was conceived for incorporation into a platform scale of the above description, and in one aspect, the invention may be considered as combined into such a scale. Platform scales of the above description are often used as and are referred to as household bathroom scales.
2. Description of the Prior Art
Such scales, as currently in production, are often provided with an electrically energized indicating system such as a lamp which illuminates a dial, or preferably such as light-emitting diode numerals in a digital scale. A scale of the latter type is exemplified, for instance, by the disclosure of U.S. Pat. No. 4,173,263 entitled "Digital Weighing Scale", invented by Frederick T. Meeks. The patent issued Nov. 6, 1979 and is assigned to the General Electric Company. As disclosed in that patent, a known method for energizing the electrical indicating system is to provide a mechanical snap-action switch which is mechanically arranged in parallel with the weighing mechanism, the switch being held open by the return springs of the weighing mechanism, and being released to the switch-closed position by the deflection and compression of the scale weighing mechanism when a weight is applied to be measured. One important problem with that arrangement is that the spring force of the switch is not totally released until the scale measures between 25 and 30 pounds. This means that the lower range of scale readings, below the point where the switch is completely released, is out of calibration, and does not read accurately. Accordingly, the weight of a small child cannnot be measured accurately. Furthermore, it is believed to be very desirable to users of bathroom scales to provide scale mechanisms having a minimum deflection, because the scale has a more solid feel. Such scales have many advantages, including compactness, and greater ease of shipment without damage. Accordingly, an effort is being made to produce platform scales having a very small total deflection of the weighing mechanism. The scale described in the above mentioned copending U.S. patent application filed by Ostrelich is such a scale.
In such scales, the total deflection of the weighing mechanism may not be sufficient to actuate the mechanical indicator switch, or so much of the deflection range of the weighing mechanism may be taken up with the deflection needed to operate the switch that virtually the entire range is made to be inaccurate by the spring force of the switch.